public void Create() { // Arrange var countSets = _sets.Count; var newSet = new SetDTO { PlannedTime = 1800, TimeForRest = 90, }; _mock.Setup(s => s.Create(It.IsAny <SetDTO>())).Returns <SetDTO>(newItem => { newItem.Id = _sets.Count + 1; newItem.CreatedDate = DateTime.Now; newItem.ModifiedDate = DateTime.Now; _sets.Add(newItem); return(newItem.Id); }); // Act var result = _setService.Create(newSet); // Assert Assert.IsNotNull(result); Assert.IsInstanceOfType(result, typeof(IOperationResult)); Assert.AreEqual(countSets + 1, _sets.Count); Assert.IsTrue(result.Succeed); Assert.IsTrue(result.ResultItemId > 0); }